PURPOSE: To decrease the number of parts by converting the output of a circuit which detects the difference between a current flowing through a ring line and a current flowing through a chip line into an absolute value output and outputting a ground button signal and a detection signal of trouble on the basis of the conversion output.

CONSTITUTION: A telephone set TEL having the ground button GB is connected to a subscriber circuit by a ground line in addition to the ring line and chip line, and when the GB is pressed, the ring line and chip line are both connected to a ground line. Both terminals of a resistance RA and both terminals of a resistance RB are connected to four input terminals of an operational amplifier OPA 3 and the output of the OPA 3 is proportional to IR∼IT, where IT is the current from the resistance RA to the chip line and IR is the current from the ring line to the resistance RB. The output of the OPA 3 is connected through a low-pass filter LPF 4 to a rectifying circuit REC 5 which converts is positively at any time. A comparative voltage V1 is connected to one input of the 1st CMP 6 to output the ground button signal detection signal and a comparative voltage V2 for detection and a comparative voltage V3 for resetting are applied to the 2nd CMP 7 to output the trouble detection signal.